The present invention relates to a device and a method for detecting the position of a movable body by using ultrasonic waves. More particularly, the present invention pertains to a device and a method that use an ultrasonic sensor to detect the position of a piston in a hydraulic cylinder used in industrial vehicles.
A typical forklift has a fork for lifting and lowering a cargo. Some forklifts are equipped with an automatic controller for lowering or lifting the fork to a predetermined position. The automatic controller requires a fork height sensor to continuously detect the height of the fork.
Reel-type fork height sensors are known in the art. A reel type sensor includes a wire, a reel for winding the wire and a rotation sensor such as a potentiometer. One end of the wire is connected to an inner mast. As the inner mast is lifted or lowered, the rotation position of the reel changes. The rotation sensor detects rotation of the reel. The height of the fork is detected based on the rotational position of the reel.
However, since the reel, around which the wire is wound, is relatively large, the reel-type sensor occupies relatively large space. Also, the wire and the potentiometer, which are exposed, can be damaged by contact with foreign objects, which may make the sensor less reliable.
Japanese Unexamined Patent Publication No. 10-238513 discloses a fork height detector that uses an ultrasonic sensor. The ultrasonic sensor is located in a lift cylinder for lifting and lowering a fork and detects the location of a piston in the lift cylinder. The height of the fork is detected based on the detected position of the piston.
Specifically, the ultrasonic sensor is located at the bottom of the cylindrical housing. The ultrasonic sensor emits ultrasonic waves to the end surface of the piston and receives the reflected ultrasonic waves. The distance between the sensor and the piston, or the position of the piston, is calculated based on the traveling time of an ultrasonic wave, or the time from when an ultrasonic wave is emitted to when the reflected wave is received. The height of the fork is calculated based on the detected piston position. Unlike reel-type sensors, the ultrasonic sensor, which is located in the lift cylinder, is not exposed. Therefore, the height detector is less vulnerable to damage, which improves reliability.
When the distance between the ultrasonic sensor and an object is relatively short, reverberation due to the emission of an ultrasonic wave remains until the reflected wave is received by the sensor. Since the sensor induces voltage signals based on the reverberation and a voltage signal based on a reflected wave, whether the reflected wave is received cannot be accurately detected based on the voltage signal outputted by the ultrasonic sensor. Thus, normally, a distance range between the sensor and one tenth of the maximum distance that can be detected by an ultrasonic sensor is out of the detectable distance range of the sensor.
If the maximum distance that can be detected by an ultrasonic sensor is, for example, ten meters as shown in FIG. 7, the detectable distance range is from a point that is one meter away from the sensor to the ten meter point. When a wave generation signal (voltage signal) is supplied to the ultrasonic sensor, the sensor generate an ultrasonic wave and induces a voltage signal S based on the reverberation of the ultrasonic wave. When the distance between the sensor and an object is shorter than one meter, the sensor induces a reception signal (voltage signal) R based on the received reflected wave while the voltage signal S is being generated. The time at which the reception signal R is generated cannot be detected based on the synthesized signal of the voltage signals S and R. Therefore, the detectable range of the ultrasonic sensor is limited.
Accordingly, it is an objective of the present invention to provide a device and a method for detecting the position of a movable body in a wide range of distance by using ultrasonic.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a device for detecting the position of a movable body that moves in a predetermined move range is provided. The device includes an ultrasonic sensor and a controller for controlling the ultrasonic sensor. The ultrasonic sensor emits ultrasonic waves toward the movable body and receives ultrasonic waves that are reflected by the movable body. The controller controls the ultrasonic sensor to consecutively emit a plurality of ultrasonic waves that have different amplitudes at predetermined intervals, thereby measuring the distance between the ultrasonic sensor and the movable body. The controller computes the distance between the ultrasonic sensor and the movable body based on a period from when the ultrasonic sensor emits one of the ultrasonic waves to when the ultrasonic sensor receives the one ultrasonic wave that is reflected by the movable body.
The present invention may also be applied to a method for detecting a position of a movable body that moves in a predetermined move range by using ultrasonic waves emitted form an ultrasonic sensor. The method includes consecutively emitting a plurality of ultrasonic waves that have different amplitudes at predetermined intervals from the ultrasonic sensor, thereby measuring the distance between the ultrasonic sensor and the movable body, and computing the distance between the ultrasonic sensor and the movable body based on a period from when the ultrasonic sensor emits one of the ultrasonic waves to when the ultrasonic sensor receives the one ultrasonic wave that is reflected by the movable body.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.